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Full text of "Mathematical And Physical Papers - Iii"

300          ON THE  CHANGE OF REFRANGIBILITY OF  LIGHT.
leaves having previously been partially dried by pressing them between sheets of blotting paper. Nettle was chosen partly because it stands boiling without losing its green colour, and partly for other reasons. My object in boiling the leaves was to obtain the green colouring matter more nearly in a state of isolation, but it seems to have the additional advantage of giving a solution less liable to decomposition. Indeed, this fluid seemed disposed to remain permanently unchanged when kept in the dark; but a small portion of it which was exposed to strong light had its colour rapidly discharged.
48. When fresh leaves are left in contact with alcohol in the dark, or in only weak light, the colour of the fluid changes by degrees, and it seems to approximate (making allowance for impurities) to a type which is nearly represented by the fluid obtained in this manner from laurel leaves, or that obtained by treating with alcohol tea leaves from which a good deal of brown colouring matter has first been extracted by water. This type was rather ideal than actual, being derived from a comparison of different cases, until it seemed to be realized in the case of a fluid obtained by re-dissolving in alcohol a crust* which had formed itself at the bottom of a test tube containing leaf-green. The principle to which the peculiar absorption and internal dispersion of such a fluid seems due may be called modified leaf-green. The fluid itself is not green but olive-coloured, becoming red at great thicknesses.
49. When solutions of leaf-green, and of its various modifica-tions, are examined in different thicknesses by the light of a candle, there are five bands of absorption which may be observed
[* From the spectrum of its solution, this crust must have been the product of decomposition by acids of the principal red-absorbing and red-iluorescing constituent of the chlorophyll mixture. This mixture consists (in land plants) mainly of two red-fluorescing and red-absorbing substances and a yellow non-iluorescent substance, all showing characteristic bands of absorption. They are all three, especially the first two, easily altered by acids. The " modified leaf-green " is the mixture with the first two substances decomposed as if by the minute quantity of acid obtained from the extract of the leaves. As the relative proportion of the three substances in the extract is liable to vary with the circumstances of the manipulation, and as tne first two are very easily altered by the least trace of acid, there is some uncertainty in endeavouring (as in  49) to identify the bands of absorption described by different observers who worked with the natural mixture.]